Bearing surface layer for magnetic motor

ABSTRACT

Magnetic motor component sleeves ( 209 ) for motor components with the sleeves ( 209 ) having various helpful magnetic characteristics, such as high magnetic permeability; magnetic saturation; residual (or remanent) magnetization; anisotropic magnetic properties. Method for making a magnetic linear motor shaft ( 202 ), including thermal treatment to temporarily change the dimensions of various shaft components to allow tight assembly.

FIELD OF THE INVENTION

The present invention is directed to the construction of magneticmotors, and more particularly to shafts for use in linear motors, andeven more particularly to shaft sleeves for doubly salient linear,magnetic motors.

BACKGROUND OF THE INVENTION

Generally speaking, a conventional magnetic motor includes two piecesthat move relative to each other. Each of the two pieces includes somemeans of generating a magnetic field. The interaction between themagnetic fields generated by each of the pieces forces the pieces tomove relative to each other. Usually, the magnetic field of at least oneof the pieces will be selectively adjusted over time so that, as therelative spatial relationship of the pieces changes over time, themagnetic fields of the respective pieces will continue to interact tocontinue to activate relative motion in a desired direction.

Usually at least one of the pieces of the magnetic motor will employ oneor more electromagnet(s), such as an electromagnetic coil, to generateits magnetic field(s). By using an electromagnetic piece, the timing ofcurrent supplied to the electromagnet(s) can be used to control thedirection and strength of the magnetic fields with respect to time. Bycarefully controlling the electromagnetic piece's magnetic field as itscounterpart piece moves, the magnetic field will pull and/or push thetwo counterpart pieces into relative motion. As the counterpart piecescontinue in their relative motion, the direction and/or magnitude of thecurrent in the electromagnet(s) can be changed so that the new magneticfield of the electromagnet(s) will continue to force the desiredrelative motion.

There are various geometries for magnetic motors. One popular geometryis the rotary motor. In the rotary motor, a rotor piece is driven torotate relative to a stator piece. Although the scope of the presentinvention may include rotary motor embodiments, this document isprimarily concerned with another popular conventional magnetic motorcalled the linear magnetic motor. Linear magnetic motors include astator piece, and a shaft member that is driven to move linearly (thatis, as a straight line translation) with respect to the stator piece.

More particularly, this document is primarily concerned with linearmagnetic motors wherein an elongated shaft member: (1) is at leastpartially surrounded by the stator, and (2) is constrained to movelinearly within the stator by a bearing. (Generally the bearing housingand stator are fixed relative to each other and can therefore be thoughtof as a subassembly.) As will be seen from the prior art embodimentdescribed below, it is difficult to make a shaft that simultaneously:(1) performs well magnetically; and (2) performs well with respect towear at the bearing.

FIGS. 1 and 2 show typical prior art linear magnetic motor 100,including shaft 102, stator 104 and bearings 106. Shaft 102 generatesmagnetic fields by virtue of having a series of built-in permanentmagnets 110. Stator 104 generates magnetic fields through a series ofannular magnetic coils 105. By timing the flow of current in the coilswith respect to the position and/or momentum of shaft 102, theinteraction of magnetic forces from the shaft and from the stator willactuate the shaft to move. More particularly, the shaft is constrained,by bearings 106, to move linearly in the direction of arrow D.

FIG. 2 shows a more detailed view of shaft 102 and one of the magneticfields that it generates. Shaft 102 includes sleeve 109, annular,permanent magnet 110, pole pieces 112 and core 114. In this assembly,maximizing the magnetic force on the shaft will tend to advantageouslymaximize the thrust of the linear motor. In order to maximize themagnetic force on the shaft, the magnetic field of permanent magnet 110should cause as much magnetic flux density as possible linking stator104 and shaft pole pieces 112.

There are several variables that control the magnitude of the fluxdensity in the vicinity of the stator. One variable is the strength ofpermanent magnet 110. For more thrust, the strength of magnet 110 shouldbe increased as much as possible and/or as much as is cost effective. Inpractice, the magnets employed as annular, permanent magnets 110 tend tobe extremely powerful permanent magnets. In fact, the permanent, annularmagnets tend to be so powerful that the heavy shaft sub-assemblies oftenneed to be handled with great caution. This is because of the tendencyfor the heavy shaft to be powerfully propelled through space due to theinteraction between the powerful magnetic field of its own magnets 110and any external magnetic field that may be present.

As shown in FIG. 2, another variable that has an influence on the fluxdensity is the size of the effective air gap G. The effective air gap isthe distance between pole piece 112 and stator 104. As shown in FIG. 2,the effective air gap G in this example is the sum of the actual air gap108 and the thickness of sleeve 109. Some effective air gap is needed toprevent the shaft from rubbing against the non load-bearing surfaces ofthe stator poles. On the other hand, decreasing this air gap, withoutentirely eliminating it, will advantageously cause the field of magnet110 to have greater flux density in the vicinity of the stator due tothe increased proximity between magnet 110 and the stator. As fluxdensity from magnet 110 in the vicinity of the stator increases,increased interaction of the magnetic fields results in increased forceon the shaft, increased attendant actuation of the shaft and increasedmotor thrust.

Yet another variable affecting magnetic flux density in the vicinity ofthe stator is the flux density located along the effective air gap. Asshown in FIG. 2, there are generally three paths A, B, C for themagnetic field of magnet 110. While magnet paths are generally circuits,it is noted that the magnetic “paths” referred to in this document referto the portion of the magnetic circuit that lies outside of the magnetitself.

Path A passes through sleeve 109, which is part of the effective airgap. Path B passes through actual air gap 108, which is also part of theeffective air gap. Path C passes through the stator. Permanent magnetsare generally limited in the maximum amount of magnetic flux that theyare capable of outputting. For an annular magnet of finite flux outputcapability, greater magnetic flux along paths A and B reduces the fluxavailable for path C. As explained above, it is flux density of path C(that is, flux that reaches the vicinity of the stator) that contributesto motor thrust.

Shifting attention to the upper portion of FIG. 2, sleeve 109 isconventionally made from materials that: (1) have a low magneticpermeability; and (2) do not exhibit significant remanent magnetization.The non-magnetic nature of sleeve 109 works to minimize flux alongsleeve 109 through path A. Nevertheless, some relatively small amount ofmagnetic flux is generally “lost” along path A. To represent this lostflux, a solitary dashed flux line is shown passing along and through thesleeve in FIG. 2. Because only a small fraction of the total flux islost along path A, a higher portion of the total flux generated bymagnet 110 will be directed through path C into the vicinity of thestator.

Not too much flux is “lost” at the actual air gap (that is, magneticpath B), either. Because actual air gap 108 is made of air, thispotential flux leakage path B has extremely low permeability (therelative permeability of air equals 1.0) and no substantial remanentmagnetization. Since the path B leakage flux is small and is primarily aresult of sleeve 109, no dashed flux lines are shown along actual airgap 108 at the upper half of FIG. 2.

Because path A leakage flux is increased by the sleeve, one may questionwhy sleeve 109 is present. One important reason for the sleeve is thatthe sleeve provides a bearing surface to slidably mate with bearing 106as bearing 106 constrains the linear motion of shaft 102. If no sleevewere present, then the permanent magnets and the intermediate polepieces of shaft 102 would contact the bearing. Because of the limitedchoice of materials that can be used to make the permanent magnets, andbecause of physical discontinuities between magnets and pole pieces, theexposed magnets would not generally provide an acceptable bearingsurface. This is due to the friction and wear characteristics that asurface of exposed magnets and pole pieces would have. Therefore, asmooth, long-wearing sleeve is generally necessary at the outer major,bearing surface of a magnetic motor shaft.

Besides providing a relatively smooth and low-friction bearing surface,sleeve 109 also helps provide structural integrity for shaft 102. Thiscan be especially important because the strong permanent magnets 110 cancreate magnetic attraction toward the stator wall sufficient to deformthe entire shaft, absent proper structural support.

Therefore, sleeve 109 is a necessary evil of sorts. Preferably, underthe conventional thinking, a material and thickness for the sleeve isselected to: (1) have a low magnetic permeability; (2) avoid magneticsaturation from the magnetic field of the shaft magnets; (3) have a lowremanent magnetization value; (4) be easy to shape; (5) be relativelyinexpensive; and/or (6) provide good bearing wear. In light of thesevarious objectives, stainless steels are often used for shaft sleeves inmagnetic motors. On the negative side, stainless steels are not theeasiest materials to work with and do not necessarily provide the lowestrate of bearing wear. On the positive side, stainless steels do performwell relative to other materials that have low magnetic permeability andlow residual magnetization. It is recognized that stainless steel is ametal with moderate wear characteristics, so sleeve 109 is constructedto be sufficiently thick to accommodate expected wear.

SUMMARY OF THE INVENTION

In addition to recognized shortcomings of conventional sleeve materials,there are other shortcomings, which may not heretofore have beenrecognized as shortcomings. By using a thick and relatively impermeablesleeve: (1) the total flux (that is, the sum of path A flux, path B fluxand path C flux) is decreased; and (2) the effective path C flux iscorrespondingly decreased. It should be kept in mind that the path Cflux passes through the sleeve in a direction normal to the sleeve'smajor inner and outer surfaces.

Also, many magnetically impermeable materials are difficult to preciselyform or shape. As a result of this, the actual air gap may be increasedto accommodate the relatively wide tolerance on the diameter of theshaft caused by imprecision in the thickness of the sleeve. Increasingthe actual air gap, to accommodate relatively large sleeve thicknesstolerances, will decrease total flux, which in turn decreases theeffective path C flux.

The present application deals with these, recognized and unrecognized,problems in the above described prior art by selecting sleeve materialswith different levels of magnetic properties (e.g., permeability,remanent magnetization, saturation, anisotropy) than is conventional.Use of these “magnetic” materials will generally result in a bearingsurface layer that is more conducive to carrying and maintainingmagnetic flux than is conventional. Although this increased sleevepermeability can nearly double the path A leakage flux between adjacentsecond magnet adjacent poles, the higher permeability also increases theuseable flux density through path C. In this way, substantially the sameor better motor thrust can be achieved. At the same time, themagnetically permeable sleeve materials can often be selected to providebetter wear at the bearing than conventional, relatively impermeablesleeve materials.

Also, the present invention involves methods for fabricating motorpieces (e.g. shafts) that include sleeves made of these differentmaterials. At least some embodiments of the present invention mayexhibit one or more of the following objects, advantages and benefits:

(1) magnetic motors with increased thrust;

(2) magnetic motors with better wear characteristics;

(3) magnetic motors with an improved relationship between magnetstrength and magnetic thrust;

(4) magnetic motors with decreased thrust losses due to decreasedfriction;

(5) easier and/or more reliable fabrication of magnetic motors;

(6) longer lasting magnetic motors;

(7) magnetic motors that require less power to be input relative toresulting thrust; and

(8) better structural support in magnetic motor shafts.

According to a first aspect of the present invention, a magnetic motorincludes a first motor assembly (e.g., a stator) and a second motorassembly (e.g., a shaft, a rotor). The first motor assembly includes afirst bearing surface layer and a first magnet. The first magnet isfixed with respect to the first bearing surface layer. The first magnetis structured to generate a first magnetic field. The second motorassembly includes a second solid bearing surface layer in the form of asleeve and a second magnet. The second bearing surface layer is locatedso that at least a portion of the second bearing surface layer is incontact with at least a portion of the first bearing surface layer. Thesecond bearing surface layer includes a material that has relativemagnetic permeability of x, wherein x is greater than 2.0. The secondmagnet is preferably fixed with respect to the second bearing surfacelayer. The second magnet is structured to generate a second magneticfield. The first and second motor assemblies are structured so thatforces caused by the interaction of the first and second magnetic fieldswill cause the first motor assembly and the second motor assembly tomove relative to each other. The first and second bearing surface layersare in moving contact to at least partially guide the relative motion ofthe first and second motor assemblies.

According to a further aspect of the present invention, a magnetic motorincludes a first motor assembly and a second motor assembly. The firstmotor assembly includes a first bearing surface layer and a firstmagnet. The first magnet is fixed with respect to the first bearingsurface layer. The first magnet is structured to generate a firstmagnetic field. The second motor assembly includes a second solidbearing surface layer and a second magnet. The second bearing surfacelayer is located so that at least a portion of the second bearingsurface layer is in contact with at least a portion of the first bearingsurface layer. The second magnet is fixed with respect to the secondbearing surface layer. The second magnet is structured to generate asecond magnetic field. The first and second motor assemblies arestructured so that forces caused by the interaction of the first andsecond magnetic fields will cause the first motor assembly and thesecond motor assembly to move relative to each other. The first andsecond bearing surface layers are in moving contact to at leastpartially guide the relative motion of the first and second motorassemblies. The second bearing surface layer has a magneticpermeability, shape and location so that at least a portion of thesecond bearing surface layer is magnetically saturated by a magneticfield of the second magnet.

According to a further aspect of the present invention, a magnetic motorincludes a first motor assembly and a second motor assembly. The firstmotor assembly includes a first bearing surface layer and a firstmagnet. The first magnet is fixed with respect to the first bearingsurface layer. The first magnet is structured to generate a firstmagnetic field. The second motor assembly includes a second solidbearing surface layer and a second magnet. The second bearing surfacelayer is located so that at least a portion of the second bearingsurface layer is in contact with at least a portion of the first bearingsurface layer. The second bearing surface layer includes a material thathas residual magnetization of x, wherein x is greater than 500 Gauss.The second magnet is fixed with respect to the second bearing surfacelayer the second magnet is structured to generate a second magneticfield. The first and second motor assemblies are structured so thatforces caused by the interaction of the first and second magnetic fieldswill cause the first motor assembly and the second motor assembly tomove relative to each other. The first and second bearing surface layersare in moving contact to at least partially guide the relative motion ofthe first and second motor assemblies.

According to a further aspect of the present invention, a magnetic motorincludes a first motor assembly and a second motor assembly. The firstmotor assembly includes a first bearing surface layer and a firstmagnet. The first magnet is fixed with respect to the first bearingsurface layer. The first magnet is structured to generate a firstmagnetic field. The second motor assembly includes a second bearingsurface layer and a second magnet. The second bearing surface layer islocated so that at least a portion of the second bearing surface layeris in contact with at least a portion of the first bearing surfacelayer. The second bearing surface layer is anisotropic in its magneticpermeability. The second magnet is fixed with respect to the secondbearing surface layer. The second magnet is structured to generate asecond magnetic field. The first and second motor assemblies arestructured so that forces caused by the interaction of the first andsecond magnetic fields will cause the first motor assembly and thesecond motor assembly to move relative to each other. The first andsecond bearing surface layers are in moving contact to at leastpartially guide the relative motion of the first and second motorassemblies.

According to a further aspect of the present invention, a method ofmaking a magnetic shaft includes several steps. A providing stepinvolves providing a stack comprising a plurality of discrete magnetshaving an initial stack diameter. Another providing step involvesproviding a tube having internal space of an initial internal diameter.A gun-drilling step involves gun-drilling the tube so that the initialinternal diameter is increased to an as gun-drilled internal diameter.An assembling step, performed after the gun-drilling step, involvesassembling the stack and the tube by inserting the stack into theinternal space of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is side, diagrammatic view of a prior art magnetic linear motor.

FIG. 2 is a more detailed, and partially cut-away view of the prior artlinear magnetic motor of FIG. 1.

FIG. 3 is a partially cut-away side view of a first embodiment of amagnetic linear motor according to the present invention.

FIG. 4 is a cross-sectional view of the shaft of the FIG. 3 embodiment.

FIG. 5 is a flowchart of a magnetic motor shaft assembly methodaccording to the present invention.

FIG. 6 is a cross-sectional view of a second embodiment of a magneticlinear motor according to the present invention.

FIG. 7 is a magnified view of a portion of FIG. 6.

FIG. 8 is a magnified view of a portion of FIG. 7.

FIG. 9 is a graph showing residual magnetization of a shaft sleeve.

FIG. 10 is a cross-sectional view of a third embodiment of a magneticlinear motor according to the present invention (cross-hatching has beenomitted from the cross-sectional view of FIG. 10 for the sake ofclarity).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before starting a description of the Figures, instructions forinterpreting the words and phrases of this patent document will beprovided. More particularly, many jurisdictions allow a patentee to actas its own lexicographer, and thereby allow the patentee to provideinstructions in a patent document as to how the words, terms and phrasesof the document are to be interpreted as a legal matter. For example, inthe United States, the prerogative of the patentee to act as its ownlexicographer has been solidly established based on statutory and caselaw. Accordingly, the following section provides rules for interpretingthe words, terns and phrases the claims of this patent document

Interpretive Rules

Rule 1: There is a “Specially Defined Terms” section set forth below.Only words, terms or phrases that are explicitly defined in theSpecially Defined Terms are to be considered to have a specialdefinition, and, of course, the explicit definition provided herein isto serve as the definition for these terms. Accordingly, sources such asthe patent specification and extrinsic evidence shall not be used tohelp define these terms—the explicitly provided definitions shallcontrol.

Rule 2: If a word, term or phrase is not specially defined, then itsdefinition shall be determined in the first instance by resort todictionaries and technical lexicons that either exist as of the timethis patent document is filed. (See definition of “dictionaries andtechnical lexicons” below in the Specially defined Terms section.) It isacknowledged that dictionaries and technical lexicons often providealternative definitions. Also, definitions provided in differentdictionaries and different lexicons often differ and are not alwaysentirely consistent. In that case, it must be decided which definitionis in best accordance with this document. Rules 3 and 4, set forthbelow, provide some guidelines for choosing between alternativedefinitions for a word, term or phrase.

Rule 3: The role of the specification (other than the Specially DefinedTerms section) as an interpretive or definitional aid shall be limitedto helping choose between alternative definitions that meet therequirements of Rule 2 (above). However, the specification will only beuseful when the specification is more consistent with one proposed,pre-existing definition than another.

Rule 4: The role of extrinsic evidence (e.g., expert witnesses) as aninterpretive of definitional aid shall be limited to helping choosebetween alternative definitions that meet the requirements of Rule 2(above). However, the extrinsic evidence will only be useful when theextrinsic evidence is more consistent with one proposed, preexistingdefinition than another.

Specially Defined Terms

the present invention: means at least some embodiments of the presentinvention; references to various feature(s) of the “present invention”throughout this document do not mean that all claimed embodiments ormethods include the referenced feature(s).

structured to: this phrase is used in the claims to indicate that something X is “structured to” perform some objective Y. This means that Xmust have appropriate structure to meet the objective Y that occursafter the “structured to” language. It does not mean that the possiblestructures for X are limited to what is shown in the specification, butrather includes any and all X, now conventional or to be developed inthe future, wherein the structure of X allows the X to perform objectiveY. (Note that X and Y are used as variables in this definition of“structured to;” in the claims, various things may be recited as the Xvariable for purposes of applying this definition, and variousobjectives may be recited as the Y variable.)

comprising . . . a; comprising . . . one; comprising . . . x: comprisingmeans including; for example, if a claim recites that an assembly“comprising a” widget, then the claim should be construed to coverassemblies that have one widget or more than one widget; the fact thatthe assembly includes a widget does not mean that covered assemblies arelimited to one widget unless such a limitation is explicitly present inthe claim.

dictionaries and/or technical lexicons: any document whose primarypurpose is the definition of words, terms and/or phrases; on the otherhand, documents that merely discuss, explain or provide examples ofdevices or methods, without purporting to provide definitions ofspecific words, phrases or terms, are not to be considered asdictionaries and/or technical lexicons.

bearing surface layer: any bearing surface conventional now, or to bedeveloped in the future, including, but not limited to bearingspermitting mutual linear motion of components, bearings permittingmutual linear translational motion of components, bushing geometrybearings, and ball bearing geometry bearings; also, bearing surfacelayer does not imply that the bearing has a discrete surface layer; forexample, a standard ball bearing would have a bearing surface layer(because its balls do have surfaces which can be conceptually viewed asthin surface layers) even if the balls are uniform and do not have anysort of coating sleeve, or other type of discontinuity defining adiscrete layer.

first magnet, second magnet: a magnet is any component that generatesone or more magnetic dipoles, magnets include, but are not limited to,permanent magnets and electromagnets.

magnetic permeability: the ratio of the magnetic flux density in amaterial to the magnetizing force producing it, referenced to the valuefor a vacuum. The permeability referred to is as tested, as applicable,according to: (1) ASTM A 342 “Standard Test Methods for Permeability ofFeebly Magnetic Materials;” or (2) ASTM A-772-89, “Test Method for ACMagnetic Permeability of Materials Using Sine Current” as applicable.

magnetic saturation: that degree of magnetization force where a furtherincrease in magnetization force produces no significant increase in themagnetic flux density (permeability) in a specimen.

shaft: includes, but is not limited to, cylindrical shafts, shafts ofpolygonal cross-section and shafts without a uniform cross-section; itis further noted that a shaft does not need to be cylindrical in orderto define a radial direction or a lengthwise direction.

residual magnetization value: the remanent magnetic induction remainingin a material after: (1) the material has been magnetically saturated byan externally-supplied magnetic field, and (2) the externally-suppliedfield has been reduced to zero subsequent to the magnetic saturation.

high thrust: a linear magnetic motor that can deliver at least 10 poundsof peak thrust per square inch of active shaft surface area.

To the extent that the definitions provided above are consistent withordinary, plain and accustomed meanings (as generally evidenced, interalia, by dictionaries and/or technical lexicons), the above definitionsshall be considered supplemental in nature. To the extent that thedefinitions provided above are inconsistent with ordinary, plain andaccustomed meanings (as generally evidenced, inter alia, by dictionariesand/or technical lexicons), the above definitions shall control. If thedefinitions provided above are broader than the ordinary, plain andaccustomed meanings in some aspect, then the above definitions willcontrol at least in relation to their broadening aspects.

DESCRIPTION OF PREFERRED EMBODIMENT(S) I. First Embodiment MagneticMotor (FIGS. 3 and 4)

FIGS. 3 and 4 show exemplary magnetic motor 200 according to the presentinvention. Magnetic motor assembly 200 includes stator 204, sleeve 209,annular permanent magnets 210, pole pieces 212, core 214 and annularspacers 218. Stator 204 is similar to prior art stator 104 and will notbe discussed in detail.

Core 214, annular permanent magnets 210, pole pieces 212, annularspacers 218 and shaft sleeve 209 are assembled to form shaft 202, asshown in FIG. 4. Core 214 provides structural support and may be madehollow or solid. Core 214 has a low magnetic permeability (e.g.,magnetic permeability of approximately 1.0). In some embodiments of thepresent invention, the core may be omitted because the stronger andstiffer sleeve materials that can now be used as a shaft sleeve mayprovide sufficient structural support for the shaft even without a core.If there is no core, the inside of the shaft may be hollow, or,alternatively, the permanent magnets and any pole pieces may be made asdisks (instead of as annular shapes).

Permanent magnets 210 are similar to prior art magnets 110 and will notbe discussed in detail. Pole pieces 212 (and pole pieces in the stator)serve to make the motor doubly salient by concentrating magnetic flux.In order to focus the flux, pole pieces 212 preferably have a magneticpermeability above 1000. For example, the pole pieces may be made frommild steel, silicon iron BFM, 1018 steel, 4130 steel or low carbon 1008steel. Preferably the pole pieces are approximately of the same axiallength as magnets 210. This way the magnetic profile of the shaft willvary in a smooth, sinusoidal fashion. When shaft 202 has a smooth,sinusoidal profile it is easier to achieve constant thrust as the motormoves under the influence of the controlled, varying magnetic field ofthe electromagnets of stator 204. This is especially true when stator204 applies 3-phase AC power to its electromagnets.

Spacers 218 are present so that the outer surface of the shaft will berelatively smooth. More particularly, annular permanent magnets oftenhave relatively large tolerances. If the magnets were nominallydimensioned so that the outer circumferences of the magnets were alignedwith the outer circumferences of the pole pieces, then there would belarge discontinuities in the outer circumferential surface of themagnet/pole piece stack due to the large magnet tolerances. Annularspacers 218 can be more precisely manufactured and maintain the magnetsin reasonable coaxial alignment between the inner circumferentialsurfaces of the spacers and core 214, as shown in FIG. 4.

Preferably, spacers 218 have a low magnetic permeability of about 1.0.This prevents lost flux through the spacers. Preferably, spacers 218 aremade of stainless steel and have a radial thickness larger than 0.02inches. Preferably, spacers are a little bit longer than the permanentmagnets in the axial direction (e.g., by 0.005 inches). As discussedbelow, spacers are not needed if the permanent magnets with tighttolerances can be used.

Exemplary motor 200 has some important differences from conventionalmagnetic motors, especially with respect to sleeve 209. Preferably,sleeve 209 is made of hard steel, for example, #4130 carbon steel,hardened to Rockwell “C” 40-50. The hard steel of sleeve 209 is strong,wear resistant and easily shapeable. While the strength, wear resistanceand shapeability of hard steel are well known, such a material waspreviously believed to be unsuitable as a material for magnetic motorsleeving because of its magnetic characteristics.

Because steels and other strong, stiff materials can be used for theshaft sleeve, the shaft sleeve may be made much thinner because it willnot wear down much (the wear may be even be negligible). However, thesleeve should be thick enough to provide structural support. If a coreis used in the shaft, then much structural support may come from thecore and the sleeve may be made very thin. On the other hand, if thecore is omitted, then the sleeve must be made thick enough to providethe structural support necessary to maintain the magnets in radialalignment. Also, the steels and other materials that can be used asshaft sleeves under the present invention should preferably be smoothand hard so that there is less friction at the bearings. The shaftsleeve material should be chosen so that it does not corrode under theinfluence of adjacent materials or the environment of the motor.

However, according to the present invention, the magneticcharacteristics typical of hard steel are considered an advantage ratherthan a disadvantage. To particularly identify some of these magneticcharacteristics, sleeve 209: (A) has a relatively large magneticpermeability; (B) has a significant residual magnetization; and (C)magnetically saturates. A discussion of each of these magneticproperties follows.

(A) Magnetic Permeability of Sleeve

According to the present invention, motor sleeves preferably have amagnetic permeability of greater than 2.0, even more preferably have amagnetic permeability greater than 10, and most preferably have amagnetic permeability greater than 100. Hard steel sleeve 209 has amagnetic permeability of over 100. This means that, for a permanentmagnet 210 of given strength and coercivity, the density of the magneticfields will generally increase because of the decreased magneticreluctance of the sleeve which forms a part of the various magneticpaths.

More particularly, the path A flux density (see FIG. 3) will be largebecause of the high permeability of the sleeve. This is notadvantageous. However, the path C flux density (see FIG. 3) will alsoincrease. This is advantageous because it is the path C flux densitythat drives the motor. Furthermore, the use of strong, wear-resistantsleeve material allows the sleeve to be made thinner. This thinnersleeve leads to many performance advantages that will be explained indetail below. At this point, suffice it to say that the increased path Cflux and the thinner sleeve advantages outweigh the fact that somewhatincreased flux is (uselessly) directed along the sleeve though path A.

(B) Residual Magnetization of Sleeve

According to the present invention, motor sleeves preferably have aresidual magnetization of greater than 500 Gauss, even more preferablyhave a residual magnetization between 1000 and 10,000 Gauss. Hard steelsleeve 209 has a residual magnetization between 1000 and 10,000 Gauss.The sleeve will remain reliably and residually magnetized in alignmentwith the permanent magnets of the shaft.

(C) Magnetic Saturation of Sleeve

Whether sleeve 209 will magnetically saturate under the influence of themagnetic field of magnets 210 will depend upon: (1) the strength of themagnets; (2) the geometry of the shaft (e.g., sleeve thickness); (3)magnetic permeability of the shaft sleeve material; and (4) magneticsaturation properties (e.g., rated saturation flux density, B_(sat)) ofthe shaft sleeve material. Given a typical high thrust magnetic linearmotor geometries (e.g., sleeve thickness of 0.015 to 0.02 inches), theshaft sleeve will saturate at magnetic permeabilities of approximately2.4 or greater. Conventional shaft sleeves do not magnetically saturate.Under the present invention, it is preferable to magnetically saturatethe shaft sleeve. If the magnetic field from permanent magnets 210 isstrong enough then sleeve 209 will magnetically saturate.

Typically, the shaft sleeve material is not likely to magneticallysaturate in the radial direction over that portion of the shaft sleevethat overlies the pole pieces and/or the magnetic poles of the permanentmagnets. Because path C passes primarily through this non-saturated zonein the vicinity of the poles, the magnetic reluctance of flux path Cwill not tend to increase due to magnetic saturation.

However, even though the portion of the sleeve in the vicinity of thepole pieces does not saturate, the portion of the sleeve that lies (withrespect to the axial direction) between the magnetic poles of eachpermanent magnet may become saturated. This is because the path A fluxis directed normally to the thin, annular cross-section of sleeve thatis available to carry the path A flux. The thin, annular cross sectiondoes not require as much flux to saturate because the flux isdistributed over such a small cross-sectional area that flux densityincreases dramatically with additional flux. This is why certainembodiments of the present invention will manifest magnetic saturationin some portions of the sleeve, but not others.

Most preferably, a shaft sleeve material may be selected so that itpossesses levels of magnetic permeability μ and magnetic saturation fluxdensity B_(sat) such that it will magnetically saturate in the regionbetween pole pieces, but will not magnetically saturate over the regionof the pole pieces. This partial saturation will limit the marginalmagnetic permeability along path A and serve to limit the flux lostalong path A. While the saturated area may impinge upon some of the pathC flux path, some or even most of path C would still cross thenon-saturated portion of the sleeve in the vicinity of the pole pieces.The partial saturation may increase the reluctance for path C, but thepartial non-saturation prevents the reluctance of path C from increasingtoo much. In this manner, magnetic saturation characteristics may beselected in such a way as to: (1) advantageously limit the flux lostalong flux path A, and (2) not unduly increase the reluctance of path C.

For a fuller appreciation of why the magnetic and wear characteristicsof sleeves of the present invention are advantages that overbear thedisadvantage of increased lost flux through the sleeve (path A), it ishelpful to understand the subtle interplay of the magneticcharacteristics and physical motor geometry of magnetic motors of thepreset invention. This interplay will now be discussed.

By comparing prior art FIG. 2 with FIG. 3, it is observed that themagnetic field of the annular magnet of both prior art magnetic motor100 and exemplary magnetic motor 200 is primarily directed along threeflux paths A, B and C. More particularly, flux path A is the flux thatis directed generally parallel to, and within the body of, therespective shaft sleeves 109, 209. Flux path B is directed generallyparallel to the respective shaft sleeves, but is located within therespective actual air gaps, located between the respective outersurfaces of the shafts and the respective inner surfaces of the stators.Flux path C represents the flux that reaches the vicinity of therespective stators 104, 204, and flux path C therefore represents theeffective flux that helps to actuate and de-actuate the respectivemotors. On the other hand, the flux of paths A and B do not serve auseful function, and this flux is wasted.

Despite the similarity of the geometry of the three flux paths A, B, Cof the respective motors, the total flux and the pattern of flux densityis quite different in prior art motor 100 than it is in motor 200.First, the difference in the pattern or distribution of flux densitywill be explained. Flux path A will have a much, much larger density inmotor 209 than in corresponding prior art motor 100. This is because ofthe high magnetic permeability of sleeve 209. The high permeability ofsleeve 209 is very conducive to induced magnetic fields (that is whatmagnetic permeability is all about), so there is a much higher fluxdensity than if a conventional sleeve, having a magnetic permeability ofless than 2, was employed. In other words, path A of motor 200 is arelatively low reluctance path that will draw a relatively highproportion of the total flux put out by the annular magnet.

Viewed in isolation, this high flux density along path A would seem todecrease the thrust of motor 200 because some of that flux would bere-directed up to the vicinity of the stator, thereby contributing tomotor thrust, if motor 200 employed the low magnetic permeabilitysleeves of the prior art. However, as explained below, this apparentdisadvantage of a high permeability shaft sleeve is overborne byadvantages, when motor 200 is considered as a whole.

Moving now to flux path B, there is not a great flux density over path Bin either motor 100 or motor 200. In each case, the air gap is fairlysmall, and, of course, the air has a small permeability and thereforehas flux path B represents a high reluctance path under bothconventional magnetic motors and those according to the presentinvention. However, there are two differences between the air gap inmotor 100 and motor 200: (1) actual air gap difference; and (2)effective air gap difference. Together and/or separately, these twodifferences have a considerable effect on motor performance. Moreparticularly, because of the novel shaft sleeve materials that are nowcontemplated by the present invention: (1) the actual air gap in motor200 may be made smaller than the corresponding actual air gap in motor100; and (2) the effective air gap in motor 200 may be made smaller thanthe corresponding effective air gap in motor 100.

The reason that the effective air gap may be made smaller in motor 200will now be explained. In motor 100, the permeability of the shaftsleeve is relatively small (generally the conventional sleevepermeability is approximately 1.0, which is the value of air or avacuum). This low, conventional permeability of motor 100 significantlyadds to the magnetic reluctance of flux path C. In fact, because theconventional low permeability sleeve of motor 100 has a reluctance thatis approximately the same as that of equal distance of air, the sleevecan be considered as part of the air gap for the purpose of determiningthe magnetic reluctance of flux path C. Thus in motor 100 the effectiveair gap equals the actual air gap plus the shaft sleeve thickness.

In motor 200, the effective air gap will tend to be much smaller. Thisis because motor 200 has a high magnetic permeability shaft sleeve. Inembodiments of motor 200 where the magnetic permeability of the shaftsleeve is extremely high, the sleeve may not significantly add to themagnetic reluctance of path C at all. In this embodiment, the effectiveair gap would be very close to equal to the actual air gap. Therefore,these embodiments of motor 200 would have a much smaller effective airgap than comparable embodiments of motor 100.

In embodiments of motor 200 where magnetic permeability of the shaftsleeve is larger-than-conventional, but not extremely large, theeffective air gap will still tend to be smaller than in comparableconventional motors 100. For example, if the shaft sleeve of motor 200were constructed to have a magnetic permeability of 2.4, then thereluctance of the shaft sleeve would contribute, in a non-negligiblefashion, to the effective air gap. More specifically, the amount ofeffective air gap attributable to the 2.4-permeability shaft sleevewould be equal to the thickness of the shaft sleeve divided by itspermeability (2.4). In other words, the amount of the effective air gapattributable to the shaft sleeve is inversely proportional to the shaftsleeve relative permeability in the direction of flux path C. As afurther example, a ferromagnetic shaft sleeve with a typical highpermeability of 1000 would add an effective air gap of 1/1000 of an inchfor every inch of sleeve thickness.

Even in embodiments of motor 200 where the contribution of the shaftsleeve to the effective air gap is not negligible, it should be borne inmind that the relationship between effective air gap and magneticreluctance of path C is an inverse linear relationship—a relativelysmall decrease in the effective air gap will result in a large decreasein the magnetic reluctance of flux path C, and a disproportionate andadvantageous increase in motor thrust, especially in doubly salientmotors with a small effective air gap.

Moving now to the actual air gap difference which may be realized as aresult of the shaft sleeve materials of the present invention, onereason for the smaller actual air gap in motor 200 is that the shaft ofmotor 200 can usually be constructed to a tighter tolerance. In both theprior art motor 100 and motor 200, it is recognized that the air gapshould be made as small as possible, with the restriction that the shaftpreferably should not touch the stator anywhere along its length. Thisis because such contact may damage the stator and/or shaft throughexcessive friction. However, if the stator is a bearing-lined statorsuch that the interior surfaces of the stator are designed to supportand guide the shaft, then shaft can and should touch the stator lining.

Therefore, it is the tolerances on the size (for example, tolerance onshaft diameter) and the shape (for example, tolerance central axis) thateffectively determine how small the air gap can indeed be. However,tolerances on the shaft (especially the axial integrity) tend to bedriven up by the fact that the powerful, multiple-dipole magnetic fieldsgenerated by the powerful annular magnets within the shaft tend to bendthe shaft a bit and otherwise deform the shaft in difficult-to-predictways.

Because the shaft of motor 200 can usually be constructed to tightertolerances, the actual air gap will usually be smaller in motor 200.There are a couple of reasons that the shaft sleeve materials of thepresent invention will generally lead to the advantageous tightertolerances. one reason is that shaft sleeve 209 in motor 200 can be (andpreferably is) made of a high magnetic permeability material. Instead ofbeing constrained to the limited palette of low magnetic permeabilitymetals, ceramics and plastics, the motor designer is now allowed to useattractive and heretofore shunned materials, such as hard steel, for theshaft sleeve. These materials will be more shapeable to begin with, and,perhaps more importantly, their high strength per unit volume canprevent the magnets from deforming the shaft. This is not to say thatall highly magnetically permeable materials have a high strength, butmerely that it may be much easier to choose an acceptable, high-strengthmaterial when one is not limited to magnetically impermeable metals. Theadditional workability and structural strength that is made possible bythe shaft sleeve materials of the present invention will often leaddirectly to tighter shaft tolerancing (and a smaller effective air gap).

Another reason for the small actual air gap is that the shaft sleeve 209may also be made to be relatively thick. The prevailing belief in thearea of magnetic motor shaft design is that the shaft sleeve must bemade as thin as possible so that the effective air gap is small.However, when using a highly magnetically permeable material, the shaftpole faces are effectively extended by the thickness of the shaftsleeve. The present invention makes it feasible to design thicker shaftsleeves. These thicker shaft sleeves will increase the structuralintegrity of the shaft, which in turn will decrease critical shafttolerances. This is an additional reason that the actual air gap will bedecreased in the present invention (without fear that the shaft willever touch the stator).

On the other hand, some designers may choose to make magneticallypermeable shaft sleeves of the present invention as thin sleeves.Basically, shaft tolerance and flux loss needs to be balanced on a motordesign by motor design basis, depending upon the required thrust,tolerances, bearing life and desired shaft sleeve material.

A smaller actual air gap results in increased effective (path C)magnetic flux and a consequent increase in motor thrust. One benefit ofa smaller actual air gapis that less flux is lost along path B, alongthe length of the air gap. A much more important advantage of a smallair gap is the fact that a small actual and/or effective air gapdrastically increases total flux. The reason that a small air gap leadsto greater total flux, and the advantage of greater total flux will bediscussed below in connection with the discussion of total flux of motor200.

Moving now to the path C component in the pattern of flux density ofmotor 200, path C is the flux that is present in the vicinity of thestator, radially outward of the actual air gap. As mentioned above theflux of path C is the portion of the magnetic field that interacts withthe magnetic fields of the stator to make the shaft and stator move (andstop moving) relative to each other. The greater the flux density alongpath C, the greater the thrust of the motor. In motor 200, the fluxdensity along path C is much greater than for comparable prior art motor100. This is because the total flux from high energy annular magnet 210is much greater than the total flux put out by comparable, high energymagnet 110.

This brings the discussion back to the issue of total flux. The fluxthat a given magnet (annular or otherwise) will put out is a function ofthe reluctance of the various paths in the vicinity of the magnet. Ifall of the paths are high reluctance, or low permeability, paths (e.g.,a magnet suspended in a vacuum), then the flux will be relatively low.On the other hand, as the reluctance of some or all of the availablepaths is decreased, the same magnet will put out much more flux, andthis increased flux will primarily be located along the low reluctancepaths.

Applying these guidelines for magnetic flux to the present situation,path C of motor 200 has a much, much lower reluctance than thecomparable path C of motor 100. For this reason, magnet 210 will outputa lot more flux than magnet 110, and this increased flux will beprimarily disposed along path C, thereby greatly increasing the thrustof the motor. The reason that path C of motor 200 is a low reluctancepath is a result of employing a different type of material to constructshaft sleeve 209. As mentioned above, the present invention contemplatesthe use of high strength, shapeable, magnetically permeable materials toconstruct shaft sleeve 209. The use of these types of materials causesthe reluctance of path C to decrease for two reasons: (1) the air gapcan be made smaller, and (2) the sleeve has a high magneticpermeability. Each of these reasons will now be discussed in order.

As mentioned above, the shaft sleeve materials that can be used underthe present invention allow tighter tolerancing for the shaft, which inturn allows a smaller air gap. By reviewing path C for motor 200 (seeFIG. 3), path C runs through the following segments (1) pole piece 212;(2) shaft sleeve 209; (3) air gap G; and (4) stator 204. The reluctancefor path C is equal to the sum of the reluctances associated with eachof these four segments of path C identified above. The reluctance of thepole piece (portion (1)) and the stator (portion (4)) is the same forboth motor 200 and comparable prior art motor 100, so the reluctance ofthese portions will not be discussed in detail. The reluctance of theshaft sleeve (portion (2)) will be discussed below.

Focusing on the reluctance of the air gap portion, the air gap has avery large reluctance which is positively correlated with its size. Inother words, decreasing the air gap decreases reluctance. Also, thedecrease of reluctance with gap size is not a linearcorrelation. Rather,small decreases in the air gap effect a tremendous, disproportionatedecrease in reluctance for this part of path C. Therefore, even thesmall decrease in gap size permitted by the tighter tolerancing cangreatly decrease path C reluctance, increase path C flux, and therebygreatly increase the motor thrust. Perhaps surprisingly, the smalldecrease in air gap permitted under the present invention will tend tomore-than-offset the useful flux lost through path B (and thecorresponding thrust lost) due to the permeable shaft sleeve.

Moving to the portion of path C that runs through the shaft sleeve (in agenerally perpendicular direction (see FIG. 3)), the reluctance of thisportion of the path is decreased because of the higher permeabilitymaterials that are used to construct the shaft sleeve under the presentinvention. In a conventional motor, such as motor 100, the impermeableshaft sleeve contributes to the path C reluctance. That is true only toa much smaller degree under the present invention.

In conventional motors, the sleeve was kept as thin as possible in orderto maximize the path C flux. After all, the shorter the length of path Cthat passed through impermeable zones, the less the reluctance and thegreater the motor thrust. On the other hand, the sleeve had to be thickenough to prevent shaft deformations and allow reasonably tighttolerancing on the shaft.

Under the present invention, the designer is presented with a couple ofnew options. First, the permeable sleeve 209 of the present inventionmay be made thin. This may be especially feasible because of the higherstrengths of some the high permeability shaft sleeve materials (e.g.,hard steel) that are available for use under the present invention.These thin sleeves would be advantageous in that they result in lessflux being lost along path A and that they would contribute extremelylittle to the reluctance of path C. On the other hand, a thin sleeve maymean that the tolerances on the shaft will remain relatively high, whichis a negative for the reasons discussed above.

Instead of opting for a thin, permeable sleeve, some designers may makethe sleeve considerably thicker than was feasible in conventionalmagnetic motors with impermeable shaft sleeves. These thick sleevesstill may not contribute to the reluctance of path C very much,especially at very high permeability values. For example, a prior sleevethat has a permeability of 1.0 and a thickness of 0.5 millimeters wouldhave about the same reluctance as a permeable sleeve with a permeabilityof 2.0 and a thickness of 1.0 millimeter. Designers are free to make thesleeves considerably thicker, while still decreasing the reluctance ofpath C relative to the non-permeable sleeve, prior art magnetic motors.

In deciding on thickness of sleeves when designing motors according tothe present invention, the motivations to make the sleeve thinner (e.g.,low reluctance) and the motivations to make the sleeve thicker (e.g.tighter tolerances, smaller air gap) will still need to be balancedagainst each other, and optimized depending upon the totality of thecircumstances. However, the various balances that can be achieved underthe present invention will generally be more favorable than the balancesthat can be achieved with prior art non-permeable shaft sleeves.

Before leaving this discussion of the portion of flux path C that passesthrough the sleeve, it is noted that the sleeve may also be made to beanisotropic in its magnetic permeability (the inverse of reluctance).More particularly, anisotropic materials possess different permeabilityvalues depending upon the relative orientation of the material and theapplied magnetic field. While many anisotropic materials may havedrawbacks in that they may be expensive or difficult to work with, tothe extent that anisotropic materials are practical, such materialscould greatly improve the thrust performance of motors of the presentinvention.

For example, the material of the shaft sleeve may be oriented so that ithas high permeability in the radial direction (direction outward fromthe centerline of the shaft) and low permeability in the axial direction(direction along the shaft). In this example, the reluctance of fluxpath C will be decreased by the radial component of the permeability. Atthe same time, the reluctance of path A (a lost flux path) will bedecreased due to the low permeability in the axial direction of thesleeve.

II. Method of Assembling a Magnetic Motor (FIG. 5)

FIG. 4 is a flowchart for a method of assembling a shaft according tothe present invention. At step S10, the core piece (preferably tubularor solid) is bolted into an appropriate fixture. Because of thepowerful, high energy magnets that are to be stacked around the corepiece, the fixture must be very secure. For example, the core piece maybe locked into a stainless steel nut.

At step S11 annular magnets, pole pieces and annular spacers are stackedaround the core piece to make a stack sub-assembly. More particularly,magnets and pole pieces are stacked directly around the core inalternating fashion. The annular spacers are placed around the outsideof the annular magnets either before or immediately after each magnet isset down around the core. The magnets are preferably oriented so thatthe poles of consecutive magnets are oriented in opposite directions onan alternating basis (as is conventional).

Preferably, (unset) epoxy is disposed between the adjacent ends of theannular magnets and pole pieces. Preferably, this is accomplished bybathing the magnets and/or pole pieces in epoxy immediately prior tostacking them around the core. Because the pole pieces are tightlytoleranced to fit closely around the core piece, the annular magnetswill be coaxially centered precisely about the core piece. The epoxyhelps keep the magnets, which are not as tightly toleranced, coaxiallycentered. As an alternative, pairs made of one annular magnet and onecore piece may be epoxied prior to insertion of the core piece. If epoxyis used, then the epoxy should be allowed to set after the stacksubassembly is completed. Also, a top nut may be used to clamp the stackin the axial direction while the stack is setting.

At step S12, the stack sub-assembly is removed from the fixture and theouter radial surface of the stack is machined down so that the stacksubassembly has a predetermined outer diameter. After machining, inorder to prepare the stack sub-assembly for assembly with the shaftsleeve, the stack sub-assembly is cooled to a cold temperature (e.g., 0degrees Fahrenheit) so that the outer diameter of the stack sub-assemblydecreases due to the thermal treatment.

In parallel with the preparation of the stack sub-assembly, the shaftsleeve is also prepared for the ultimate assembly of the shaft. At stepS20, hardened tube stock is provided for the shaft sleeve and cut toappropriate length. At step S21, the shaft sleeve is gun-drilled to afairly precise inner diameter. At step S22, the shaft sleeve isheat-treated and honed so that the inner diameter is very precise. Atthis juncture, the inner diameter of the shaft sleeve should be equal toor slightly less than the outer diameter of the stack sub-assembly. Atthis juncture, the shaft sleeve and the (not yet cooled) stacksub-assembly would have a very close fit—so close that they could not beassembled.

At step S23, the shaft sleeve is heated (e.g., to 275 degreesFahrenheit) so that the shaft sleeve and its inner diameter expand. Whenthe stack sub-assembly is cooled and the shaft sleeve is heated, thenstack subassembly is inserted (at step 30) into the inner diameter ofthe shaft sleeve because of the respective thermal contraction andexpansion.

The thermal treatment allows the stack assembly to be inserted in theshaft sleeve. As an alternative method, only the stack assembly may bethermally treated (that is, cooled), or only the shaft sleeve may bethermally treated (that is, heated). However, the thermal treatmentsmust effect sufficient, temporary geometrical adjustment so that thestack assembly will fit within the shaft sleeve. Of course, after thethermal treatment is over, the shaft sleeve and stack assembly willshrink and expand back toward their respective pre-thermal treatmentsizes, which will result in an extremely tight interference fit.

It is noted that other ways exist, besides thermal treatment, totemporarily adjust the relative sizes of the stack assembly and/or theshaft sleeve to allow assembly. For example, the inner axial cavity ofthe shaft sleeve may be pressurized to expand the inner diameter. Afterthe stack assembly is inserted into the shaft sleeve, the pressure wouldbe released to allow the shaft sleeve to contract back down to a verytight fit.

At step S31, the outer radial surface of the shaft assembly is matchedand finished to a final condition (preferably by a grinder or lathe) andfitted to the bearing to make a linear motor. This assembly process ispreferred for the magnetic sleeve embodiments of the present invention,but this inventive process may also be used for assembling moreconventional shafts. The gun-drilling allows a precise, relatively longand axially aligned bore to be made. This is important so that thesleeve fits tightly over the stack to provide precise alignment ofinternal components and enhanced structural integrity.

III. Second Embodiment of a Magnetic Motor (FIGS. 6 to 9)

As shown in FIGS. 6 to 8, motor 300 includes stator 304 and shaft 302.Shaft 302 is driven to move in a linear direction under the guidance ofbearings 306. As shown in FIG. 7, shaft 302 includes shaft sleeve 309,alternating magnets 310 and pole pieces 312. Sleeve 309, is made of hardsteel so that it has large magnetic permeability and residualmagnetization and exhibits magnetic saturation. Because of the magneticcharacteristics of shaft sleeve 309, it does not contribute to theeffective air gap G. Rather, as shown in FIGS. 7 and 8, the effectiveair gap G corresponds to the tiny actual air gap between the outercircumferential surface of shaft 302 and the inner circumferentialsurface of stator 304.

Similar to motor 200, motor 300 has flux paths A (lost flux), B (lostflux) and C (effective flux). Similar to motor 200, the path A flux isrelatively large, but is more than made up for by the decreased magneticreluctance of path C resulting from the smaller effective air gap G.

A simplified analytical representation shows how the highly permeableshaft sleeve 309 improves motor performance. Specifically, motor thrustas a function of the various reluctance paths of the motor is given bythe following roughly accurate equation (1):F=K ₁/(1_(s)/μ_(s)+1_(g)/μ₀ +R _(m))−K ₂×(μ_(s)×1_(s))  (1)In this equation, F is the motor thrust, K₁ is a constant ofproportionality for thrust due to path C magnetic flux, K₂ is a constantof proportionality for thrust lost due to lost magnetic flux, 1s is theshaft sleeve thickness, μ_(s) is the shaft sleeve permeability, 1_(g) isthe air gap length, u₀ is the permeability of air (approximately 1.0)and R_(m) is the magnet effective reluctance. For most practical motorgeometries, K₂ is small relative to K₁. When K₂ is small, as theequation shows, us can be made relatively large. This increases thrustdue to the K₁ term. Not much force is lost because the K₂ term is small.

FIG. 9 shows a graph representing the residual magnetization in theradial direction of shaft sleeve 309. In the graph, the H axisrepresents the net magnetic field that to which the shaft sleeve isexposed. This field will be the net field of the permanent magnet andthe opposing (but variable) magnetic field of the electromagnets of thestator. The B axis represents the magnetization of the shaft sleeve.

As shaft sleeve 309 is first applied to shaft 302, the net magneticfield increases from 0 to a high value as shown by curve 402. At thesaturation point H₀ of shaft sleeve 309, the magnetization will stopincreasing even as the applied magnetic field H increases. As stator 304applies oppositely directed magnetic fields, the net magnetic field issubject to decrease, as shown by curve 404. If the net field H fallsbelow the saturation point H₀, then the magnetization of shaft sleeve309 may decrease slightly. However, even if the net magnetic field Hfalls to 0, shaft sleeve 309 will still be magnetized with its residualmagnetization B_(f), as shown in FIG. 9. If the net magnetization H wereto fall below 0, then the magnetization of shaft sleeve 309 would fallaccording to curve 406, but this would seldom, if ever, happen in amagnetic motor because the stator magnets (negative H contribution) arenot that powerful compared to the permanent magnets (positive Hcontribution).

As shown in FIG. 8, shaft 302 does not include annular spacer piecesbetween magnet 310 and shaft sleeve 309. This is because more preciselydimensioned permanent magnets are used in shaft 302. However, bothmagnets 310 and pole pieces are bathed in 2 part thermal setting epoxyresin as they are being assembled onto the core. When the shaft is bakedto set the epoxy a thin (e.g., 0.001 to 0.002 inches) layer of epoxy(not shown) surrounds magnet 310, including the outer circumferentialsurface of magnet 310 between the magnet and shaft sleeve 309. The epoxysurrounding the magnet and the pole pieces prevents air from beingtrapped under shaft sleeve 309. The epoxy layer effectively smoothesdiscontinuities in the outer circumferential surface of the magnet/polepiece stack to prevent bubbles and bumps in the shaft sleeve. It isimportant to have a smooth shaft sleeve to ensure even wear at thebearings and to prevent mechanical interference with the stator.

As shown in FIGS. 7 and 8, stator 304 includes stator coil 350, dustseal 352 and stator liner 354. Coil 354 is one of the electromagnets,whose magnetic fields drive shaft 302 in linear motion. Dust seal 352serves to protect coil 354 from dust. Stator liner 354 is preferablymade of stainless steel and also serves to protect the stator. Statorliner 354 is extremely thin so that it does not add significantly to theeffective air gap.

IV. Third Embodiment of a Magnetic Motor (FIG. 10)

FIG. 10 shows another preferred embodiment of a linear magnetic motor500. Motor 500 includes stator 504, shaft 502 and bearing 506. Bearing506 is preferably a bronze bushing.

Shaft 502 includes magnets 510, pole pieces 512, annular spacers 518,shaft sleeve 509 and nut assembly 570. Magnets 510, pole pieces 512,annular spacers 518 and shaft sleeve are similar to their respectivelycorresponding components in previously-discussed motor 200. In thisembodiment, the axial length of two magnets 510 and two pole pieces(“the pole pair pitch”) is preferably 0.922 inches. The nut assembly 570is structured to accommodate a cap (not shown) by a threaded connection.

Stator 504 includes coils 556, power supply lines 557, pole pieces 558,spacer rings 560, tensioning rod 562 and collar assembly 564. Coils 556are preferably 115 turn, 19 gauge copper wire. Power supply linespreferably selectively provide 3 phase AC power to the coils to controlmovement of shaft 502 relative to the stator. Pole pieces 558 serve toconcentrate and direct the magnetic field of coils 556 by forming a lowreluctance path. Accordingly, pole pieces 558 are preferably made of amaterial with a high magnetic permeability. Tensioning rod 562 holds thestator stack together through a threaded engagement, as shown in FIG.10. Collar assembly 564 is a housing for stator 504.

V. Conclusion

U.S. Pat. No. 5,691,582, issued on Nov. 25, 1997 to Lucas, et al.(hereby incorporated by reference), discloses a doubly salient linearmagnetic motor having an especially preferred construction due to itsuse of high energy magnets and the highly favorable layout of its fluxpaths. Modifying this prior art design so that its shaft sleeve is madeof a magnetically permeable, low friction, long wearing material (e.g.,hard steel) yields an especially preferred embodiment of the presentinvention.

The description and examples set forth in this specification andassociated drawings set forth preferred embodiment(s) of the presentinvention. The specification and drawings are not intended to limit theexclusionary scope of this patent document. Many designs other than theabove-described embodiments will fall within the literal and/or legalscope of the following claims. Because it is generally impossible for apatent to describein its specification every conceivable and possiblefuture embodiment of the invention, the exclusionary scope of thispatent document should not be limited by features: (1) reflected in thespecification and/or drawings, but (2) not explicated or reasonablyimplicated by the language of the following claims.

1. A magnetic motor comprising; a first motor assembly comprising: afirst bearing surface layer, and a first magnet, fixed with respect tothe first bearing surface layer, structured to generate a first magneticfield; and a second motor assembly comprising: a second magnet, and asecond solid bearing surface layer in the form of a sleeve, located sothat at least a portion of the second bearing surface layer is incontact with at least a portion of the first bearing surface layer, withthe second bearing surface layer comprising a material that has relativemagnetic permeability of x, wherein x is greater than 2.0; said assemblystructured to generate a second magnetic field defined by at least saidsecond magnet and said second bearing surface layer, with the first andsecond motor assemblies being structured so that forces caused by theinteraction of the first and second magnetic fields will cause the firstmotor assembly and the second motor assembly to move relative to eachother, and with the first and second bearing surface layers being inmoving contact to at least partially guide the relative motion of thefirst and second motor assemblies.
 2. The motor of claim 1 wherein themagnetic motor is a high thrust magnetic motor.
 3. The motor of claim 1wherein x is greater than
 100. 4. The motor of claim 1 wherein: thefirst motor assembly is a stator; the first bearing surface layercomprises a bushing; the first magnet is an electromagnet, such that thefirst magnetic field can be selectively controlled; the second motorassembly comprises a shaft; the second bearing surface layer is locatedover at least a portion of the shaft; and the second magnet locatedwithin the shaft and comprises at least one permanent magnet.
 5. Themotor of claim 4 wherein the motor is a doubly salient motor.
 6. Themotor of claim 4 wherein the shaft comprises: a plurality of annular,pennanent magnets; a plurality of pole pieces, with the magnets and thepole pieces being assembled in an alternating manner; and a sleevedisposed at least partially around the alternating magnets and polepieces, with the sleeve comprising an outer major surface, and with thesecond bearing surface layer being located at least partially along theouter major surface of the sleeve.
 7. The motor of claim 1 wherein thesecond bearing surface layer comprises hard steel.
 8. The magnetic motorof claim 1, wherein the material of the second bearing layer has aresidual magnetization value of y that is greater than 500 Gauss.
 9. Themotor of claim 8 wherein y is greater than 1000 Gauss.
 10. The motor ofclaim 8 wherein: the first motor assembly is a stator; the first bearingsurface layer comprises a bushing; the first magnet is an electromagnet,such that the first magnetic field can be selectively controlled; thesecond motor assembly comprises a shaft; the second bearing surfacelayer is located over at least a portion of the shaft; and the secondmagnet located within the shaft and comprises at least one permanentmagnet.
 11. The motor of claim 10 wherein the motor is a doubly salientmotor.
 12. The motor of claim 10 wherein the shaft comprises: aplurality of annular, permanent magnets; a plurality of pole pieces,with the magnets of the pole pieces being assembled in an alternatingmanner; and a sleeve disposed at least partially around the alternatingmagnets and pole pieces, with the sleeve comprising an outer majorsurface, and with the second bearing surface layer being located atleast partially along the outer major surface of the sleeve.
 13. Amagnetic motor comprising: a first motor assembly comprising; a firstbearing surface layer, and a first magnet, fixed with respect to thefirst bearing surface layer, structured to generate a first magneticfield; and a second motor assembly comprising: a second solid bearingsurface layer, located so that at least a portion of the second bearingsurface layer is in contact with at least a portion of the first bearingsurface layer, and a second magnet, fixed with respect to the secondbearing surface layer, structured to generate a second magnetic field,with the first and second motor assemblies being structured so thatforces caused by the interaction of the first and second magnetic fieldswill cause the first motor assembly and the second motor assembly tomove relative to each other, and with the first and second bearingsurface layers being in moving contact to at least partially guide therelative motion of the first and second motor assemblies; wherein thesecond bearing surface layer has a magnetic permeability, saturationcharacteristic, shape and location so that at least a portion of thesecond bearing surface layer is magnetically saturated by a magneticfield of the second magnet.
 14. The motor of claim 13 wherein the secondbearing surface layer comprises: a saturated portion that ismagnetically saturated by the magnetic field of the second magnet; andan unsaturated portion that is not magnetically saturated by themagnetic field of the second magnet.
 15. The motor of claim 14 wherein:the saturated portion comprises a portion of the second bearing surfacelayer that is located in the vicinity of the second magnet, between thepoles of the second magnet; and the unsaturated portion comprises aportion of the second bearing surface layer that is located in thevicinity of the poles of the second magnet.
 16. The motor of claim 13wherein: the first motor assembly is a stator; the first bearing surfacelayer comprises a bushing; the first magnet is an electromagnet, suchthat the first magnetic field can be selectively controlled; the secondmotor assembly comprises a shaft; the second bearing surface layer islocated over at least a portion of the shaft; and the second magnetlocated within the shaft and comprises at least one permanent magnet.17. The motor of claim 16 wherein, during normal operation of the motor,a portion of second bearing surface layer proximate to poles of the atleast one permanent magnet are magnetically unsaturated and a portion ofthe second bearing surface layer located between the poles ismagnetically saturated.
 18. A magnetic motor comprising: a first motorassembly comprising: a first bearing surface layer, and a first magnet,fixed with respect to the first bearing surface layer, structured togenerate a first magnetic field; and a second motor assembly comprising:a second solid bearing surface layer, located so that at least a portionof the second bearing surface layer is in contact with at least aportion of the first bearing surface layer, with the second bearingsurface layer being anisotropic in its magnetic permeability, and asecond magnet, fixed with respect to the second bearing surface layer,structured to generate a second magnetic field, with the first andsecond motor assemblies being structured so that forces caused by theinteraction of the first and second magnetic fields will cause the firstmotor assembly and the second motor assembly to move relative to eachother, and with the first and second bearing surface layers being inmoving contact to at least partially guide the relative motion of thefirst and second motor assemblies.
 19. The motor of claim 18 wherein:the first motor assembly is a stator; the first bearing surface layercomprises a bushing; the first magnet is an electromagnet, such that thefirst magnetic field can be selectively controlled; the second motorassembly comprises an elongated shaft defining a lengthwise directionand a radial direction; the second bearing surface layer is located overat least a portion of the shaft; and the second magnet located withinthe shaft and comprises at least one permanent magnet.
 20. A magneticmotor comprising: a first motor assembly comprising: a first bearingsurface layer, and a first magnet, fixed with respect to the firstbearing surface layer, structured to generate a first magnetic field;and a second motor assembly comprising: a second bearing surface layer,located so that at least a portion of the second bearing surface layeris in contact with at least a portion of the first bearing surfacelayer, with the second bearing surface layer being anisotropic in itsmagnetic permeability, and a second magnet, fixed with respect to thesecond bearing surface layer, structured to generate a second magneticfield, with the first and second motor assemblies being structured sothat forces caused by the interaction of the first and second magneticfields will cause the first motor assembly and the second motor assemblyto move relative to each other, with the first and second bearingsurface layers being in moving contact to at least partially guide therelative motion of the first and second motor assemblies; wherein: thefirst motor assembly is a stator; the first bearing surface layercomprises a bushing; the first magnet is an electromagnet, such that thefirst magnetic field can be selectively controlled; the second motorassembly comprises an elongated shaft defining a lengthwise directionand a radial direction; the second bearing surface layer is located overat least a portion of the shaft; and the second magnet located withinthe shaft and comprises at least one permanent magnet; and wherein: amagnetic permeability of the second bearing surface layer in the radialdirection is y; a magnetic permeability of the second bearing surfacelayer in the lengthwise direction is x; and y is greater than x.
 21. Themotor of claim 20 wherein y/x is greater than or equal to 1.5.